CONFIG_SYS_HAS_CPU_MIPS64_R6 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_SYS_HAS_CPU_MIPS64_R6 (not configurable)

configname: CONFIG_SYS_HAS_CPU_MIPS64_R6

Linux Kernel Configuration
└─>CPU selection
└─>SYS_HAS_CPU_MIPS64_R6
In linux kernel since version 3.10 (release Date: 2013-06-30)  
is selected by
CONFIG_CPU_MIPS64_R6